/Engine/Source/Runtime/Core/Public/Math/TransformCalculus2D.h
Represents a 2D rotation as a complex number (analagous to quaternions). Rot(theta) == cos(theta) + i * sin(theta) General transformation follows complex number algebra from there. Does not use "spinor" notation using theta/2 as we don't need that decomposition for our purposes. This makes the implementation for straightforward and efficient for 2D.
template<typename T> class TQuat2